б)5\7(0,21-1,4x)-4\9(0,36-4,5x)=1

5/7*21/100-5/7*14/10х-4/9*36/100+4/9*45/10х=1
3/20-2/2х-4/25+2х=1
х=1-3/20+4/25
х=1-0,15+0,16
х=1,01
проверка 5/7(0,21-1,4*1,01)-4/9(0,36-4,5*1,01)=5/7*(-1,204)-4/9*(-4,185)=-0,86+1,86=1